How to get Custom SW version number of android

I have a Marshmallow device. It has custom built SW.

I have seen the build.prop file. There I can perceive the Software version name comes from ro.custom.build.version

My question is - how can I get the "ro.custom.build.version" information programmatically in my application ?

Answer

I got the answer. It is very simple -
First, I created a function to read from SystemProperties.

public String getSystemProperty(String key) {
    String value = null;

    try {

        value = (String) Class.forName("android.os.SystemProperties")
                .getMethod("get", String.class).invoke(null, key);
    } catch (Exception e) {
        e.printStackTrace();
    }

    return value;
}

And then called the function with "ro.custom.build.version" as a key

getSystemProperty("ro.custom.build.version");

Special thanks to @sasikumar for giving me the hint